1. Field of the Invention
The invention relates to a control method for a lithium ion secondary battery, and a lithium ion secondary battery system.
2. Description of the Related Art
In recent years, there has been proposed a lithium ion secondary battery that uses a lithium-transition metal composite oxide with an olivine structure, expressed by a composition formula of LiFePO4, or the like, as a positive electrode active material, and that uses a carbon-based material as a negative electrode active material (see Japanese Patent Application Publication No. 2003-36889 (JP-A-2003-36889) and Japanese Patent Application Publication No. 2006-12613 (JP-A-2006-12613)). The lithium-transition metal composite oxide with an olivine structure, expressed by LiFePO4, or the like, has a substantially constant charge and discharge potential when charged and discharged. The charge and discharge potential remains substantially unchanged even when lithium ions are desorbed or absorbed. This is because the lithium-transition metal composite oxide with an olivine structure, expressed, for example, by LiFePO4, enters a two-phase coexistence state of LiFePO4 and FePO4 when Li is absorbed or desorbed.
Thus, by using a positive electrode active material, such as LiFePO4, that undergoes charge and discharge in a two-phase coexistence state, it is possible to construct a lithium ion secondary battery with less variations in input density and output density against a variation in state of charge, and with stable output characteristics. In recent years, a research for using such a lithium ion secondary battery as a driving source for a hybrid vehicle has been conducted.
Incidentally, the above lithium ion secondary battery has a characteristic such that a negative electrode potential increases in the late stage of discharging. JP-A-2003-36889 and JP-A-2006-12613 suggest that copper is used as the material of a negative electrode current collector. However, the negative electrode current collector made of copper (copper foil, or the like) dissolves as the negative electrode potential increases to about 1.2 V. After that, a short circuit (internal short circuit) occurs between the positive and negative electrodes because of precipitation of dissolved copper, and this may shorten the service life. For this reason, it has been necessary to control the negative electrode potential so that the negative electrode potential does not increase to a dissolution potential at which the negative electrode current collector dissolves.
When the lithium ion secondary battery is used as a power source for a hybrid vehicle, electric power stored in the lithium ion secondary battery is consumed by, for example, an electronic equipment (a battery controller, an air conditioner, an audio equipment, and the like) mounted on the vehicle even when the hybrid vehicle is stopped (when the hybrid vehicle is not running but is runnable, and when an engine is not operating). Therefore, when the hybrid vehicle is continuously stopped for a long period of time in a state where the negative electrode potential has reached a value close to the dissolution potential of the negative electrode current collector, there is a possibility that the negative electrode potential increases with a reduction in the amount of electric charge stored, and, as a result, the negative electrode potential reaches the dissolution potential of the negative electrode current collector.